With increasing requirements for suppressing reactive powers and harmonics, various electric power quality compensation systems have been widely used. These electric power quality compensation systems perform real-time detection on external power networks through external detection modules and internal current monitor so as to compensate electric power qualities of the external power networks.
FIG. 1 illustratively shows an application example of an electric power quality compensation system in conventional technologies. In FIG. 1, the electric power quality compensation system 600 includes an external detection module 1, a compensation module 40 and a conventional current monitor 30. The external detection module 1 detects a current of an external power network 2 in real time, while the external power network 2 includes a power grid 21 and a non-linear load 22, for example. The non-linear load 22 may be a harmonic source, for example. The compensation module 40 and the non-linear load 22 are connected in parallel with the power grid 21 in order to compensate the harmonics of the non-linear load 22.
The conventional current monitor 30 may include one single internal monitor unit or a plurality of internal monitor units. The compensation module 40 may include one or more compensation units. The conventional current monitor 30 is connected with an output side of the external detection module 1 through an external cable 6. The compensation module 40 may realize a good filtering effect by distributing output(s) of the compensation unit(s) based on the current measured by the internal monitor unit(s) in the conventional current monitor 30.
However, the conventional current monitor has following problems.
The external detection module 1, with output of which being connected with the conventional current monitor 30, has an output load including an equivalent impedance of the external cable 6 provided between the conventional current monitor 30 and the external detection module 1 and an equivalent impedance of the measurement loop in the conventional current monitor 30. If the output load of the external detection module 1 goes beyond the output-load capacity of the external detection module 1, the current will be detected by the external detection module 1 inaccurately, and thereby the compensation effect of the electric power quality compensation system 600 will be deteriorated.
For example, the external detection module 1 may be an external current transformer comprising a primary side as the input of the external detection module 1 and a secondary side as the output of the external detection module 1 respectively. The primary side of the external current transformer is connected in series between the power grid 21 and the non-linear load 22, and the secondary side is connected with the conventional current monitor 30. As shown in FIG. 2, the secondary-side load of the external current transformer consists of two parts: one is the equivalent impedance Z6 of the external cable 6, and the other one is the equivalent impedance of the measurement loop of the conventional current monitor 30. The latter includes the equivalent impedance Z32 of the internal monitor unit(s), and further includes the equivalent impedance Z31 of the internal connection wires among the internal monitor units when there are pluralities of internal monitor units. The equivalent impedance Z6 of the external cable 6 may be expressed by an equation: Z6=resistivity of the external cable 6×length of the external cable 6/sectional area of the external cable 6. The equivalent impedance Z31 of the internal connection wires may be expressed by an equation: Z31=resistivities of the internal wires×lengths of the internal wires/sectional areas of the internal wires. It can be seen that, the more the number of the internal monitor units is, and the longer the internal wires' lengths are when the sectional areas are kept unchanged, the greater the equivalent impedance of the detection loop of the conventional current monitor 30 will be.
In general, an external current transformer has a small secondary-side capacity. For example, a current transformer with a transformation ratio of 500/5 or less usually has a secondary-side capacity of 5 VA (volt·ampere) or less, and a current transformer with a transformation ratio of 1000/5 or less usually has a secondary-side capacity of 10 VA or less. If an external current transformer has a large load which is likely to go beyond the secondary-side capacity, the current will be detected by the external current transformer inaccurately, and thereby the subsequent compensation effects of the compensation modules will be deteriorated.
To avoid the above-mentioned problem of the over large output load of the external detection module deteriorating the accuracy, conventional solutions include adjusting the distance between the current monitor and the external detection module to reduce the output-side load of the external detection module, or limiting the equivalent impedance of the measurement loop of the current monitor in the capacity of the external detection module. However, one of the above two solutions will restrict the geographical location of the current monitor, and the other will limit the selective scope of the current monitor. Thus, both of the two solutions are not ideal.